Plastic lenses, generally, are produced by injection molding, injection compression molding or so. In the conventional injection molding for plastic lenses take a process, using an injection molding-machine, to inject a melt resin of a cylinder into a metal mold applied with a mold tightening force, and then take it out thereof after cooling. In injection compression molding, a melt resin is injected into a metal mold like the above, followed by cooling while compressing it at a portion inside the cavity. Injection compression molding is effective particularly for lenses having a ameter of 50 mm or more or those thick-walled (5 mm or more), and often used in molding video projector lenses and laser-scanning optical system fθ lenses. Because of the capability of molding at an injection pressure lower than the usual injection molding, plastic lenses can be produced less in inner-diameter strain but higher in accuracy (JP-A-7-148795 (page 2, FIG. 1)).
However, in the case of a plastic lens, the lens molded possibly deviates in its outer diameter from the design value due to errors, etc. caused upon making the metal mold. Where the lens molded has an outer diameter not meeting the design value, there occurs a trouble of optical-axis deviation due to a gap caused with a mirror frame to fit the lens therein or of strains induced in the lens due to excessive tightness. For this reason, where the lens outer diameter is not formed meeting the design value, it is a practice to modify the metal mold at the entire portion of the lens circumference while taking account of the deviation, and to conduct again a molding into a lens outer diameter meeting the design value.
However, the modification to the metal mold, in a manner correcting the entire circumferential form of the lens after once molding the lens, results in an increased extra process and considerably troublesome operation, thus leading to cost increase and deadline missing.